Method of imaging using a liquid crystal display device

ABSTRACT

Binary optics are used in an illumination system to provide a method of imaging for a color projection display. In one embodiment a broad spectrum light source illuminates a multilevel optical phase element which disperses the broad spectrum light from the light source by diffraction. A display having a number of pixel elements, each capable of transmitting a predetermined spectral region, is positioned within the near field region of the multilevel optical phase element so as to receive the light dispersed by the multilevel phase element.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.08/565,058, filed Nov. 30, 1995 now U.S. Pat. No. 5,889,567, which is acontinuation-in-part application of the U.S. patent application Ser. No.08/545,990, “Illumination System for Transmissive Light Valve Displays”filed by Gary J. Swanson on Oct. 20, 1995 now abandoned on Jun. 20,2000, which is a continuation-in-part of U.S. patent application Ser.No. 08/443,180, filed May 17, 1995, which is a continuation-in-partapplication of U.S. patent application Ser. No. 08/330,339, filed Oct.27, 1994 now abandoned on Sep. 25, 1998, the teachings of which areincorporated herein by reference in their entirety.

GOVERNMENT SUPPORT

This invention was made with government support under contract numberF19628-85-C-0002 awarded by the Air Force. The government has certainrights in the invention.

BACKGROUND OF THE INVENTION

Currently used techniques for color projection displays tend to berelatively inefficient in their light utilization. Such low efficiencylimits the brightness of the display, which in effect limits theacceptable amount of ambient lighting in a viewing environment.

In certain presently used designs, light from a spectrally broad sourceis collected by a condensing lens and illuminates a spatial lightmodulator system. The spatial light modulator system comprises atwo-dimensional array of pixels and the amount of light transmittedthrough each pixel is controlled electronically. A projection lens thenimages the array of pixels on a viewing screen, the magnification of thedisplayed image being determined by the particular characteristics ofthe projection lens. The light impinging on each pixel of the spatiallight modulator is spectrally broad (i.e., white light). Therefore,unless the system is modified to distinguish colors, the display is onlycapable of displaying black and white images.

In many current systems used to modify such a system so that it iscapable of displaying color images, each pixel of the spatial lightmodulator is divided into three sub-pixels having equal areas. Each ofthe three sub-pixels is covered with a micro-color filter having adifferent spectral transmittance. For example, the filters are chosensuch that one filter transmits only red light, another filter only greenlight, and the third filter only blue light. The transmittances of thethree sub-pixels of each pixel of the spatial light modulator can becontrolled independently, resulting in the ability to display a colorimage.

The inefficiency of the approach can be seen by considering thefollowing factors. The light illuminating a full pixel essentially iswhite light and, consequently, the light impinging each sub-pixel isalso white light. The red filtered sub-pixel transmits only red light,absorbing all of the incident green and blue light. Likewise, the othertwo sub-pixels transmit only its corresponding color, absorbing theother two colors. It is apparent that this approach utilizes, at most,only one third of the available light impinging on the modulator, andabsorbs the rest.

Furthermore, state-of-the-art microcolor filters required to produceacceptable color images are approximately only 33% efficient intransmitting the color that they are designed to transmit. Therefore theoverall light utilization of current color projection displays is about10%.

One approach for improving the efficiency of color projection displaysis found in U.S. Pat. No. 5,161,042 issued on Nov. 3, 1994 to H. Hamoda.In accordance therewith, the spectrally broad input light is supplied tothree dichroic mirrors which reflect three different color components,e.g., red, green, and blue, in different directions, i.e., at differentangles with respect to each other. The reflected components are thensupplied to an array of lenses for focusing the different colorcomponents so as to converge light beams of similar wavelength rangesfor transmission through a liquid crystal display element so as to formcombined color images on a display screen. A further U.S. Pat. No.5,264,880, issued on Nov. 23, 1993 to R. A. Sprague et al., discloses asimilar approach to that of Hamoda wherein the dichroic mirrors arereplaced by a phase grating for dispersing the color components of lightreceived thereat into a spectrum of different colors at different anglesrelative to each other.

While such approaches can be used, the losses of energy of each colorcomponent are sufficient to reduce the efficiencies of such systems andto show the need for further improvement in such display systems. Suchimproved display systems should minimize such losses so as to providefor substantially the total use of the received energy across the colorspectrum in the imaging display process resulting in an improvement ofthe efficiency of the system.

SUMMARY OF THE INVENTION

The invention relates to a color projection display in which receivedlight, having a relatively broad spectrum, illuminates a multi-leveloptical phase grating so as to disperse each of the color componentscontained therein into a plurality of different diffraction orders. Inone embodiment, the diffraction orders of each color component are thenfocussed onto a zero-order phase shift element which phase shifts onlythe undiffracted light (i.e., the zero diffraction order) with respectto the diffracted light (i.e., the higher level diffraction orders). Theoutput of the zero-order phase shifter is then imaged onto a displayhaving a plurality of pixels, each pixel having sub-pixel regionsassigned to transmit different color components of light. The depths ofthe phase grating element and the zero-order phase shifter are suitablyselected so they are practical for manufacture and so the area ofchromaticity space for the color components at the image plane ismaximized.

The use of such a combination of multi-level phase grating and azero-order phase shifter, having suitably determined depths, providesdesired color components at each pixel in which essentially little or noenergy is lost. These color components are then suitably combined toprovide a color image at each of the pixels of the display which isconsiderably brighter than that available using prior known systems.

In another embodiment, a broad spectrum light source illuminates amultilevel optical phase element which disperses the broad spectrumlight from the light source by diffraction. A display having a number ofpixel elements, each capable of transmitting a predetermined spectralregion, is positioned within the near field region of the multileveloptical phase element so as to receive the light dispersed by themultilevel phase element. In one embodiment, the multilevel phaseelement is periodic in two dimensions, thereby concentrating the lightin two dimensions.

In yet another embodiment, a method for displaying a color image isdisclosed. The method for displaying a color image includes illuminatinga multilevel optical phase element with a broad spectrum light source.The multilevel phase element disperses light from the light source bydiffraction. A display having a plurality of pixel elements, eachtransmitting a predetermined spectral region, is positioned within thenear field region of the multilevel optical phase element to receive thedispersed light from the multilevel optical phase element.

Preferred embodiments of the invention include transmissive activematrix liquid crystal display devices. An active matrix array oftransistor circuits and pixel electrodes is bonded to an opticallytransmissive substrate by a layer of adhesive material. A layer ofliquid crystal material is disposed over the active matrix array to forma transmissive display structure such that actuation of the pixelelectrodes controls transmission of light through a respective volume ofthe liquid crystal material. A diffractive optical element is alsoaligned with the active matrix array to disperse light of differentcolors through different volumes of the liquid crystal material. Theactive matrix array and the diffractive optical element are mountedwithin a display or projector housing to form a direct-view orprojected-view display device.

Preferred embodiments of the invention also include an array of lightreflective pixel electrodes such as digital micromirror display devices.In a digital micromirror display device, a matrix housing contains adigital micromirror array of light reflective, electromechanical pixelsand a diffractive optical element. The diffractive optical element isaligned with the digital micromirror array to disperse light ofdifferent colors onto different electromechanical pixels. Instead ofelectromechanical pixel electrodes, a reflector liquid crystal displaydevice can be used.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the invention, including various noveldetails of construction and combination of parts, will now be moreparticularly described with reference to the accompanying drawings andpointed out in the claims. It will be understood that the particularillumination system for color display panels embodying the invention isshown by way of illustration only and not as a limitation of theinvention. The principles and features of this invention may be employedin varied and numerous embodiments without departing from the scope ofthe invention.

FIGS. 1 and 1A are block diagrams of an embodiment of a display systemusing the technique of the invention;

FIG. 2 is a perspective diagram of an embodiment of the multileveloptical phase element;

FIG. 2A is a perspective diagram of another embodiment of a multileveloptical phase element of the invention;

FIG. 3 shows a graph of optimized phase grating depths of three phaselevels for a normalized pixel dimension for red, green and blue colorchannels;

FIG. 4 shows the effective phase grating depths of three phase levelsfor a normalized pixel dimension for the wavelengths of the red, greenand blue color components;

FIG. 5 shows the percent efficiencies of the spectral content for thered, green and blue color components;

FIG. 6 shows the area of the chromaticity space covered when using aparticular embodiment of the invention on a standard 1976 CIEchromaticity graph space;

FIG. 7 is a block diagram of another embodiment of a display system ofthe invention;

FIG. 8 is a block diagram of yet another embodiment of a display systemof the invention; and

FIGS. 9 and 9A are block diagrams of a multilevel phase element and itscomplex conjugate, respectively.

FIG. 10 a flow chart illustrating a lithographic method of fabricatingthe phase plates.

FIG. 11 a flow chart illustrating a direct writing method of fabricatingthe phase plate.

FIG. 12 is a flow chart illustrating a deposition method of fabricatingthe phase plates.

FIG. 13 is a graphical representation of a constant amplitude wavefrontinterfacing with an aperture in a mask element.

FIG. 14 is a schematic diagram of a transmissive display systememploying a mask element.

FIG. 15 is a foreshortened cross sectional schematic diagram of themasked plate 110 of FIG. 14.

FIGS. 16A-16B are schematic front views of the masked plate 110 of FIG.14.

FIG. 17 is a perspective cross sectional schematic diagram of anotherpreferred embodiment of the invention employing a reflective box lightsource as a backlight.

FIGS. 18A-18B are foreshortened cross sectional views of the maskedplate 210 of FIG. 17.

FIG. 19 is a schematic diagram illustrated a preferred embodiment of adisplay system having a light pipe.

FIG. 20 is a schematic diagram of a preferred embodiment of a displaysystem having multiple light sources.

FIG. 21 a schematic diagram of a preferred embodiment of a displaysystem having a changed aspect ratio.

FIG. 22 is a schematic diagram of a preferred embodiment of a displaysystem having a multiple aperture light pipe in accordance with theinvention.

FIG. 23 is a partial cross-sectional view of an active matrix colordisplay device in accordance with the invention.

FIG. 24 is a schematic diagram of a preferred embodiment of theinvention embodied in a display system having a folded light pipeillumination system.

FIG. 25 is a schematic block diagram of a preferred embodiment of theinvention embodied in a projection display system having a sphericalreflector light source.

FIG. 26 is a schematic diagram of a preferred embodiment of theinvention embodied in a projection monitor.

FIG. 27 is a schematic diagram of another preferred embodiment of theinvention embodied in a projection monitor.

FIG. 28 is a schematic diagram, shown partially in cross-section, of apreferred embodiment of the invention embodied in a direct-viewhead-mounted display device 950.

FIG. 29 is a schematic diagram, shown partially in cross-section, of apreferred embodiment of the invention embodied in a projectionhead-mounted display device 970.

FIG. 30 is a schematic diagram, shown partially in cross-section, of apreferred embodiment of the invention embodied in a projection displaysystem having a virtual light source.

FIG. 31 is a schematic diagram of a preferred embodiment of theinvention embodied in a display system having a digital micromirrordevice (DMD).

FIG. 32 is a schematic diagram, partially in cross-section, of apreferred embodiment of the invention embodied in a DMD projectionsystem having a light pipe.

FIG. 33 is a schematic diagram, partially in cross section, of apreferred embodiment of the invention embodied in a reflective liquidcrystal display system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

To increase the light utilization of color displays, the novel techniqueof the invention can be considered, in a conceptual sense, aseffectively concentrating all of the light of each color component in areceived spectrally broad light onto appropriate sub-pixel regions at acolor image plane. For example, all of the incident red light isconcentrated in a manner such that it only illuminates the sub-pixelregions corresponding to the red component thereof, all of the incidentgreen light is concentrated in a manner such that it only illuminatesthe sub-pixel regions corresponding to the green component thereof, andall of the incident blue light is concentrated in a manner such that itonly illuminates the sub-pixel regions corresponding to the bluecomponent thereof. By so doing, the use of micro-filters is notnecessary, and the theoretical light utilization efficiency of suchtechnique approaches 100% (ignoring light loss through the system opticsby reflection and absorption).

The invention achieves such light utilization based on a techniquereferred to as aperture filling. Aperture filling is described, forexample, in the article, “Aperture filling of phase-locked laser arrays”by G. J. Swanson et al., Optics Letters, Vol. 12, April 1987. Thisarticle describes a method for increasing the energy in the central lobeof a far-field pattern of a phase-locked laser array. In accordance withthe invention, the underlying physics of this technique is modified andextended in a unique manner to solve the color display problem of lightutilization.

In accordance with the basic physics behind aperture filling, a binaryamplitude grating (a grating having a transmittance of 1 or 0) with afill factor (the ratio of the transmitting area to the total area) ofgreater than or equal to 0.25, has, aside from a phase shift of the zeroorder, a Fourier transform identical to that of a binary phase gratinghaving the same fill-factor as the binary amplitude grating. By placinga zero-order phase shift element in the transform plane of a focalimaging system, the light from an aperture with a fill-factor of ≧0.25can be uniformly spread out to fill the entire aperture. Further, byinvoking reciprocity, light from a uniform aperture can be concentratedto produce an underfilled aperture with a fill-factor of ≧0.25.

In making use of such concepts for improving the color projectiondisplay efficiency, the above phenomenon can be modified tosubstantially improve the light throughput thereof. A system embodyingthe technique in accordance with the invention is shown in FIGS. 1 and2, wherein a multi-level, e.g., a three-level, phase grating isilluminated with a spectrally broad light from a source 10, such as atungsten halogen bulb or a xenon arc lamp. Alternatively, the lightsource may comprise three separate color component sources. For example,three light emitting diodes (LEDs) or three laser sources, each emittinga separate color such as red, green, and blue color components. For thepurposes of the particular description of a preferred embodiment of theinvention, it is assumed that the illuminating source 10, whether asingle broad spectrum source or separate color sources, primarilyincludes color components of the three wavelength regions, e.g., red,green, and blue. The lateral dimension of each phase level, in oneembodiment, is assumed to be equal to the lateral dimension of asub-pixel region of the spatial light modulator. For illustrativepurposes only, FIG. 1A shows only two greatly magnified grating periods,each having corresponding three phase depth levels, occupying the entireaperture. It should be understood that a large plurality of gratingperiods, each corresponding to a pixel of the overall color image, wouldnormally occupy an aperture.

If it is assumed that a first phase depth level measured with respect toa second phase depth level at each grating period of the phase grating11 is equal to an integral number of wavelengths of red light plusone-third of such wavelength, i.e. (m+0.33), where m is an integer, andthe third phase depth level, again measured with respect to the secondphase depth level, is an integer multiple of the wavelength of redlight, the red light that is illuminating a three-level phase gratingwill in effect encounter a binary phase grating with a fill-factor of33%, and a phase depth of 0.33 wavelengths. The red light will bedispersed from the phase grating 11 into a zero diffraction order and aplurality higher level positive and negative diffraction orders whichare focussed on a zero-order phase shifter 13 via lens 12. If the zerodiffraction order (undiffracted) is then phase shifted by about 0.33wavelengths of red light by phase shifter 13, the red light exiting thesystem will be concentrated via a lens 14 so as to fill only 33% of theoutput imaging plane 15 (FIG. 1A).

The same methodology as applied above to the red light range can also beapplied to the green and blue light ranges. The second phase depth levelat each grating period equals zero wavelengths of green light bydefinition, and the first and third phase depth levels equal (n−0.33)and (n′−0.33) wavelengths of green light, respectively, wherein n and n′are integers. The green light illuminating the phase grating 11 alsoeffectively encounters a binary phase grating with a fill-factor of 33%,and a phase depth of 0.33 wavelengths. If the zero diffraction order isalso effectively shifted by about 0.33 wavelengths, the green lightexiting the system is concentrated so as to fill the 33% of the outputimaging plane that is adjacent to the 33% of the output plane occupiedby the red light (FIG. 1A).

For the blue light, the phase depth of level of each grating period,again measured with respect to the second phase depth level, equals(p′+0.33) wavelengths of blue (where p′ is an integer), and the firstphase depth level is an integer multiple of wavelengths of blue light.The blue light illuminating the grating also in effect encounters abinary phase grating with a fill-factor of 33%, and a phase depth of0.33 wavelengths of blue light. If the zero diffraction order is alsoeffectively shifted by about 0.33 wavelengths, the blue light exitingthe system is concentrated so as to fill the remaining 33% of theimaging plane not occupied by the red light and the green light (FIG.2).

The above conditions for three discrete wavelengths can in theory be metto any level of accuracy. However, in practice, the accuracy is limitedby the physical depths of the grating levels that can be practicallymanufactured. Furthermore, the system can be designed to operate overthe entire visible spectrum, rather than at only three discretewavelength regions.

The area of chromaticity space spanned by a particular embodiment of theinvention depends on the relative depths of the three phase levelregions of each grating period corresponding to each pixel, and thedepth of the zero-order phase shifter. Since the phase depths arerelative, and measured with respect to the second phase depth level, thesecond phase depth level is zero by definition, thereby leaving threevariables: the depths of phase levels 1 and 3 with respect to phaselevel 2, and the depth of the zero order phase shifter. These threeparameters in effect define the performance of the overall system, withthe measure of performance being defined as the area of chromaticityspace that is so covered. The three depth parameters are most easilyoptimized by performing a “global search” process that spans the rangeof practicable manufacturable depths. The goal thereof is to selectrelative depths which will maximize the area and the location of thespanned chromaticity space. An approach to such process is discussedbelow.

In considering the first phase level of the grating period, the phaseshifts (in waves) φ_(R) ¹,φ_(G) ¹ and φ_(B) ¹ of the red, green, andblue light can be expressed as: $\begin{matrix}{\Phi_{R}^{1} = {\frac{d_{1}}{\lambda_{R}}\quad \left( {\eta - 1} \right)}} \\{\Phi_{G}^{1} = {\frac{d_{1}}{\lambda_{G}}\quad \left( {\eta - 1} \right)}} \\{\Phi_{B}^{1} = {\frac{d_{1}}{\lambda_{B}}\quad \left( {\eta - 1} \right)}}\end{matrix}$

where η is the index of refraction of the phase grating, and d₁, is thedepth of the first phase level with respect to the second phase level.As mentioned above, it is desired that the phase shift φ_(R) ¹=m+0.33,while the phase shift φ_(G) ¹=n−0.33, and the φ_(B) ¹=p, where m, n, andp are all integers.

In a similar manner at the third phase level, having a depth of d₃ withrespect to the second phase level, the phase shifts are: $\begin{matrix}{\Phi_{R}^{3} = {\frac{d_{3}}{\lambda_{R}}\quad \left( {\eta - 1} \right)}} \\{\Phi_{G}^{3} = {\frac{d_{3}}{\lambda_{G}}\quad \left( {\eta - 1} \right)}} \\{\Phi_{B}^{3} = {\frac{d_{3}}{\lambda_{B}}\quad \left( {\eta - 1} \right)}}\end{matrix}$

Here, it is desired that the phase shift φ_(R) ³=m′, the phase shiftφ_(G) ³=n′−0.33, and the phase shift φ_(B) ³=p′+0.33, where m′, n′, andp′ are all integers.

Because the first and third phase levels of the grating are referencedin depth to the second phase level of the grating, by definition, d₂=0,and at the second phase level the phase shifts at all three wavelengthsis zero:

φ_(R) ²=0

φ_(G) ²=0

φ_(B) ²=0

In addition, at the zero-order phase shifter having a depth of d₄, aphase shift of about one-third wavelength of each color is required sothat at the phase shifter: $\begin{matrix}{\Phi_{R}^{4} = {\frac{d_{4}}{\lambda_{R}}\quad \left( {\eta - 1} \right)}} \\{\Phi_{G}^{4} = {\frac{d_{4}}{\lambda_{G}}\quad \left( {\eta - 1} \right)}} \\{\Phi_{B}^{4} = {\frac{d_{4}}{\lambda_{B}}\quad \left( {\eta - 1} \right)}}\end{matrix}$

where φ_(R) ⁴=r+0.33, φ_(G) ⁴=s+0.33 and φ_(B) ⁴=t+0.33 (where r, s, andt are integers).

Since the depths of d₁, d₂, d₃, and d₄ must be within practicalmanufacturable limits, the following practical limitations can beimposed thereon:

−5 μm≦d₁≦+5 μm

−5 μm≦d₃≦+5 μm

−5 μm≦d₄≦+5 μm

and the value of η can be assumed at a conventional value, for example,of 1.5.

Using the above equations, those in the art can then utilize a wellknown global search algorithm technique, in which the values of thedepths d₁, d₃, and d₄ are changed in steps, δd, of approximately 0.01μm, and used to determine in each case the area of the chromaticityspace that can be spanned for each set of parameters. The depths d₁, d₃,and d₄ for the solution providing a maximized area can then be used asthe practical physical depths for the three phase level regions at eachphase grating period and the practical physical depth of the zero-orderphase shifter 13.

In accordance with a specific embodiment of the invention, such aprocess was used to determine the three optimum depth parameters for asystem operating with a uniform spectral source covering a 0.40-0.68 μmwavelength region, using both multilevel phase grating and zero-orderphase shift substrates assumed to have an index of refraction of 1.5.Exemplary results for optimized sub-pixel phase grating depths of anexemplary pixel having a normalized pixel dimension are shown in FIG. 3,with the red channel having a phase grating depth 16 of 1.84 μm relativeto the green channel, and the blue channel having a phase grating depth17 of 4.0 μm relative to the green channel.

To illustrate how such an optimized phase grating design conforms to thetheory described above, the following three discrete wavelengths can beconsidered: red=0.66 μm, green=0.54 μm, and glue=0.46 μm. The effectivephase grating depths (modulo one-wave) of the three sub-pixels at thesethree phase level regions are shown in FIG. 4, where the solid line 18represents red, the dashed line 19 represents green, and the dot-dashline 20 represents blue. It should be noted that in the first sub-pixelregion, the phase grating depth for red is approximates one-thirdwavelength of red light, and the phase grating depths for green and blueare essentially zero. Similarly, in the second sub-pixel region, theeffective phase grating depth for the green approximates one-thirdwavelength of green light, and the phase grating depths for red and blueare approximately zero. In the third sub-pixel region, the effectivephase grating depth for blue approximates one-third wavelength of bluelight, while the phase grating depths for red and green areapproximately zero.

The optimized depth for the zero-order phase shifter 13 is 0.36 μm,which depth corresponds to 0.27 wavelengths of red, 0.33 wavelengths ofgreen, and 0.39 wavelengths of blue. For this example, it is noted thatthe optimum phase depth is less than one wave for all three wavelengths.FIG. 2 is a perspective view of a multilevel phase element which repeatsperiodically in the x-direction. FIG. 2A is a perspective view of amultilevel phase element which repeats periodically in both the x and ydirections. Such a configuration permits the incident light to becompressed both in the x-direction, as in the prior embodiment, and alsoin the y-direction. Methods for forming such multilevel phase elementsare well known to those skilled in the art. In particular a method forforming such multilevel diffractive optical elements is disclosed inU.S. Pat. No. 4,895,790 to Swanson and Veldkamp, the teachings of whichare incorporated herein by reference.

The system's ability to concentrate the visible spectrum into threecolor channels is illustrated in FIG. 5 for the above-mentioned 0.4-0.68μm wavelength region. The solid curve 21 represents the percentefficiency of the spectral content of the red channel, the dashed curve22 represents the percent efficiency of the spectral content of thegreen channel, and the dash-dot curve 23 represents the percentefficiency of the spectral content of the blue channel. It should benoted that the red channel efficiency peaks at the wavelength of 0.66μm, the green channel efficiency peaks at 0.54 μm, and the blue channelefficiency peaks at 0.46 μm. The red channel has a secondary peak in thefar blue region of the spectrum. This blue light, in effect “leaking”into the red channel, tends to limit the area covered in chromaticityspace. In some cases, it may be desired or required to remove thisunwanted blue light from the red channel by conventionally filtering thered channel and such removal can be achieved with a blue-blockingmicro-filter, albeit at the cost of losing a minimal amount of the bluelight energy.

As is well known to the art, the spectral content of these three colorchannels can then be used to determine the area of chromaticity spacespanned by the system. FIG. 6 shows a standard 1976 CIE chromaticityspace graph 25 which is well known to the art. The area of thechromaticity space spanned by the embodiment discussed above is depictedby three vertices of a triangle, defined by the plus signs, in thegraph. This area will be covered using essentially 100% of the receivedsource illumination.

Another embodiment of the invention does not require either thezero-order phase plate 13 or the auxiliary optics 12, 14 of the previousembodiment shown in FIG. 1. In this embodiment a broad spectrum lightsource illuminates a multilevel optical phase element which dispersesthe broad spectrum light from the light source into diffraction orders.A modulation display, having a number of pixel elements, each capable oftransmitting a predetermined spectral region, is positioned within thenear field region of a multilevel optical phase element so as to receivethe light dispersed by the multilevel phase element. In this embodiment,the free-space propagation of light from the multilevel phase elementproduces a ⅓ wavelength phase shift of the undiffracted light withrespect to the diffracted light. Because of this, the phase plate 13 andauxiliary optical elements 12, 14, which were required to produce thesame zero-order phase shift in the previous embodiment, are not requiredin this embodiment.

To understand how free-space propagation provides the required ⅓wavelength zero-order phase shift, assume that the amplitudetransmittance of the phase grating is expressed as:${t(x)} = {a_{o} + {\sum\limits_{n}{a_{n}{\exp \left\lbrack {\quad 2\pi \quad \frac{n}{T}\quad x} \right\rbrack}}}}$

where T is the grating period, a₀ is the amplitude of the undiffractedlight, a_(n) is the amplitude coefficients of the various diffractedorders, and n is an indexing parameter.

If a unit amplitude plane wave illuminates this phase grating, the lightamplitude distribution, U_(z), at a distance Z from the grating plane isdescribed by:${U_{z}(x)} = {a_{o} + {\sum\limits_{n}{a_{n}{\exp \left\lbrack {\quad 2\pi \quad \frac{n}{T}\quad x} \right\rbrack}{\exp \left\lbrack {{- }\quad {\pi\lambda}\quad \frac{n^{2}}{T^{2}}\quad Z} \right\rbrack}}}}$

where λ is the wavelength of the unit amplitude plane wave. Theirrelevant constants have been omitted from this equation. Thus thefree-space propagation over the distance Z has the effect of introducingphase shifts to the diffracted components with respect to theundiffracted component.

A distance, Z_(⅓), is defined by the equation:$Z_{1/3} = \frac{2\quad T^{2}}{3\quad \lambda}$

where λ is the central wavelength of the spectral distribution.

Substituting this equation for Z_(⅓) into X in the previous equation,results in the following light distribution:${U_{z_{1/3}}(x)} = {a_{o} + {\sum\limits_{n}{a_{n}{\exp \left\lbrack {\quad 2\pi \quad \frac{n}{T}\quad x} \right\rbrack}{\exp \left\lbrack {{- }\quad \frac{2}{3}\quad \pi \quad n^{2}} \right\rbrack}}}}$

The resulting phase shift (for all values of n that do not result in aninteger when divided by 3) is equal to an integer number of wavelengthsplus ⅓ wavelength. Because the integer number of waves of phase shiftare irrelevant, all of the values of n (that do not result in an integerwhen divided by 3) effectively see a ⅓ wave phase shift with respect tothe undiffracted light. For values of n that do result in an integerwhen divided by 3, the result is an integer number of wavelengths ofphase shift. However, for the grating described above, all the values ofa_(n) (for n divisible by 3) are zero.

Thus, the net result of free-space propagation over the distance Z_(⅓)is to produce a light distribution where the undiffracted light isphase-shifted by ⅓ wavelength with respect to the diffracted light. Itis at this location that the modulation display, such as a liquidcrystal light modulator, is placed. With such a positioning, no phaseshift element 13 or additional optics 12, 14 are needed.

It should be noted that the propagation distance Z_(⅓) is a function ofwavelength. Therefore, the free-space propagation just discussed isstrictly accurate at only one wavelength. However, acceptableperformance over the whole visible spectrum may be achieved by choosingthe Z_(⅓) distance to correspond to the wavelength at the center of thespectrum. That is, Z_(⅓) should be chosen such that$\frac{2\quad T^{2}}{3\quad \lambda_{long}} < Z_{1/3} < \frac{2\quad T^{2}}{3\quad \lambda_{short}}$

where λ_(long) is the longest wavelength of interest and λ_(short) isthe shortest wavelength of interest.

The above analysis assumes that the illumination source is a pointsource at infinity, resulting in plane wave illumination. If theillumination source is such that the approximation of a point source atinfinity which was just discussed is not valid, an embodiment whichdescribes a physically extended illumination source must be considered.

In this embodiment, shown in FIG. 7, an illumination source 10′ has adimension (for purposes of discussion referred to as the x-dimension) ofS_(c). A condensing lens 30 having a focal length of F_(c), ispositioned adjacent the illumination source 10′, at a distance of F_(c).This configuration results in an angular source extent in thex-dimension of θ_(c)≈S_(c)/F_(c).

A figure of merit for performance is the ratio b/T, where b, the blur,is the physical extent of the light pattern at the display location inthe x-dimension and T is the grating period. For good performance, thisratio is less (typically much less) than ⅓. If b is given by theexpression b =z_(⅓)θ_(c), the figure of merit becomes:$\frac{b}{T} = {\frac{2\quad T}{3\quad \lambda_{o}}\quad \theta_{c}}$

where λ_(o) is the center wavelength. For example, if an acceptablefigure of merit is ⅙, the center wavelength is 0.55 micrometers, and thegrating period is 48 micrometers, the resulting value for the angularsource size is θ_(o)=2.9 milliradians. Thus, for a 50 mm focal lengthcondensing lens, a source with a size no larger than 145 μm in thex-dimension is required for this figure of merit to hold. Hence, asource 10′ with a physical dimension smaller than most commerciallyavailable broad-spectrum incoherent sources should be used to obtaingood performance. Although broad spectrum incoherent sources of thedimension just described are not generally available, three spatiallycoherent monochromatic sources, such as three different wavelength LED'sor laser diodes, could readily be used as the illumination source 10′.As discussed with respect to the previous example, the modulationdisplay is preferably placed at a distance Z_(⅓) from the multilevelphase element 11.

Referring to FIG. 8, an embodiment which increases the extended sourceperformance includes a lenslet array 40 (in one embodiment cylindricallenslets), placed between the condensing lens 30 and the multilevelphase element 11. The focal length of each lenslet 42 is F_(m), and thedistance between the lenslet array 40 and the multilevel phase element11 is Z_(s)+F_(m). Thus, Z_(s) is the distance between the imaged source22 and the multilevel phase element 11. Each lenslet 42 focusses animage 44 of the extended source, S_(c), at a distance F_(m) from thelenslet array 40. Each of these imaged sources 44 is of physicaldimension, S_(m), in the x-dimension, where S_(m)=(F_(m) S_(c))/F_(c),centered about the optical axis of the respective lenslet 42.

For an image 44 of dimension S_(m) that lies on the optical axis of thecondensing lens, a Fresnel diffraction calculation indicates that thelight amplitude distribution at a distance Z from the multilevel phaseelement 11 is given by the expression: $\begin{matrix}{U_{z{(x)}}^{\prime} = \quad {\exp \left\lbrack {{\quad \quad}\frac{\pi}{\lambda}\quad \frac{x^{2}}{Z + Z_{s}}} \right\rbrack}} \\{\quad \left\lbrack {a_{o} + {\sum\limits_{n}{{\exp \left\lbrack {{- }\quad \pi \quad \lambda \quad \frac{n^{2}}{T^{2}}\quad \frac{Z_{s}Z}{Z + Z_{s}}} \right\rbrack}{\exp \left\lbrack {{- }\quad 2\quad \pi \quad \frac{n}{T}\quad \frac{Z_{s}}{Z + Z_{s}}\quad x} \right\rbrack}}}} \right\rbrack}\end{matrix}$

in which the irrelevant constant factors have been neglected. The firstexponential term in the series is the wavefront curvature introduced bythe lenslet 42. The first exponential term after the summation signrepresents the phase shifts incurred by the various diffraction orders.Again, what is desired is for all of the phase shifts for value of nwhich do not result in integers, when divided by three, be equal to ⅓wave. For this to be the case, Z must be given by the expression:$Z = \frac{2\quad T^{2}Z_{s}}{{3\quad \lambda \quad Z_{s}} - {2\quad T^{2}}}$

Hence, in a manner similar to the case without lenslets, the value of zshould be chosen such that:$\frac{2\quad T^{2}Z_{s}}{{3\quad \lambda_{long}Z_{s}} - {2\quad T^{2}}} < Z < \frac{2\quad T^{2}Z_{s}}{{3\quad \lambda_{short}Z_{s}} - {2\quad T^{2}}}$

wherein T is the periodicity of said multilevel optical phase element,Z_(s) is equal to the distance between said multilevel optical phaseelement and said lenslets minus the focal length of said lenslets,λ_(long) is the longest wavelength of interest and λ_(short) is theshortest wavelength of interest.

Comparing this relationship with the relationship previously shown forZ_(⅓) (the optimum distance for the plane wave illumination case):$\frac{Z_{1/3}}{Z} = {1 - \frac{2T^{2}}{3\quad \lambda \quad Z_{s}}}$

As Z_(s) approaches infinity, the distance, Z, approaches Z_(⅓) as isexpected. For finite source distance Z_(s), the optimum Z distance isgreater than the plane wave distance Z_(⅓).

The last exponential term indicates that the period of the lightdistribution at the optimum Z distance is no longer equal to the periodof the original phase grating. In effect, free-space propagation from asource 10′ located a finite distance from the grating 11 results in amagnification. This magnification, M, is given by the equation:$\frac{1}{M} = {1 = \frac{Z}{Z_{s}}}$

Note that for a finite source definition, Z_(s), the magnificationfactor is greater than one.

The angular source size of the extended source 10 as seen at the grating11, S_(m)/Z_(s). This angular source size results in a new blurdimension, b_(n), given by the expression:$b_{n} = {{\frac{S_{m}}{Z_{s}}\quad Z} = {\frac{2\quad T^{2}}{{3\lambda \quad Z_{s}} - {2T^{2}}}\quad S_{m}}}$

Because of the magnification described above, the new period of theimage pattern is, T_(n):$T_{n} = {{MT} = \frac{3\lambda \quad Z_{s}T}{{3\lambda \quad Z_{s}} - {2T^{2}}}}$

The resulting fractional blur of the image pattern can now be describedby the relationships:$\frac{b_{n}}{T_{n}} = {{\frac{2T}{3\lambda \quad Z_{s}}\quad S_{m}} = {\frac{2T}{3\lambda \quad Z_{s}}\quad \frac{F_{m}}{F_{c}}\quad S_{c}}}$

The fractional blur, with the lenslet array 40 in position, can bedirectly related to the fractional blur, b/T, without the lenslet array40, according to the expression:$\frac{b_{n}}{T_{n}} = {\frac{b}{T}\quad \frac{F_{m}}{Z_{s}}}$

This relationship clearly shows that the blurring can be dramaticallyreduced by the proper introduction of the lenslet array 40. Thereduction factor of the blurring is the ratio F_(m)/Z_(s).

So far, consideration has only been given to the one lenslet 42 centeredon the optical axis (OA) of the condensing lens 30. Additional lensletsplaced adjacent to the original lenslet 42 behave in a manner identicalto that described above for the original lenslet 42. However, because ofa coherent interaction between the light traversing different lenslets,an additional constraint is placed on the allowable center-to-centerspacings of the lenslets. The minimum center-to-center spacing distanceof the lenslets, L, is given by the expression:$L = {{pT}\left( \frac{Z + Z_{s}}{Z} \right)}$

where p is a positive integer.

In another embodiment, the multilevel phase element 11′ shown in FIG. 9,and discussed above with respect to FIG. 3, includes a double stephaving an aggregate height of 4.0 μm. The first step is 1.84 μm measuredfrom the base of the phase element and the second step is 2.16 μmmeasured from the top of the first step to the top of the phase element.A phase element constructed with these dimensions functions as describedabove.

If instead of the phase element 11′ shown, a complex conjugate phaseelement 11″ as shown in FIG. 9A is constructed, the complex conjugatephase element 11″ performs equivalently to the phase element 11′. Thereason for referring to phase element 11″ as a complex conjugate phaseelement becomes readily apparent if the complex conjugate phase element11″ is placed adjacent the phase element 11′ such that the steps arealigned. Light passing through both phase elements is unaffected, andthus, just as the integration of a wavefunction by its complex conjugateequals one, phase element 11″ acts as the complex conjugate to phaseelement 11, thereby permitting the incident light to pass both elementsthrough unaffected.

Unlike the phase element 11′, the complex conjugate phase element 11″Z_(⅓) is defined by: $Z_{1/3} = \frac{T^{2}}{3\quad \lambda}$

wherein λ is the central wavelength of the spectral distribution.

Acceptable performance over the whole visible spectrum may be achievedby choosing the Z_(⅓) distance to correspond to the wavelength at thecenter of the spectrum.$\frac{T_{2}}{3\quad \lambda_{long}} < Z_{1/3} < \frac{T^{2}}{3\quad \lambda_{short}}$

where λ_(long) is the longest wavelength of interest and λ_(short) isthe shortest wavelength of interest. Similarly, when lenslets areemployed in conjunction with the complex conjugate phase element 11′, Zmust be chosen such that:$\frac{T^{2}Z_{s}}{{3\quad \lambda_{long}Z_{s}} - T^{2}} < Z < \frac{T^{2}Z_{s}}{{3\quad \lambda_{short}Z_{s}} - T^{2}}$

wherein T is the periodicity of said multilevel optical phase element,Z_(s) is equal to the distance between said multilevel optical phaseelement and said lenslets minus the focal length of said lenslets,λ_(long) is the longest wavelength of interest and λ_(short) is theshortest wavelength of interest. Thus, using a complex conjugate phaseelement permits a closer spacing than is permitted by the non-complexconjugate phase element.

The above-described phase plates can be fabricated using a number oftechniques including, but not limited to, lithography, direct writing,deposition, diamond turning, grating ruling engine, or laser ablation.

FIG. 10 is a flow chart illustrating a lithographic method offabricating the phase plates. The phase plate is preferably fabricatedusing a lithographic mask along with standard photoresist and etchingtechniques used in semiconductor fabrication processes. Preferably, anoptical substrate (which will become the phase plate) is coated with alayer of photoresist. A lithographic mask is then used to selectivelyexpose the photoresist over desired areas of the substrate. Thephotoresist is then developed, where it is removed from the exposedareas. The substrate is then etched by one of many etching processes(e.g., reactive ion etching, ion milling, chemical wet etch) to thedesired depth of one of the phase levels. The residual photoresist isthen washed off, leaving the substrate surface with two phase levels.Repeating the above procedure for a second lithographic mask results ina substrate having three phase levels. The process can be continueduntil the desired number of phase levels are achieved.

FIG. 11 is a flow chart illustrating a direct writing method offabricating the phase plate. Direct writing is a variation of the abovefabrication procedure to expose the photoresist. Direct writing refersto the process whereby an exposing beam, such as electron or laser beam,is scanned across the surface of the substrate. The beam is turned on oroff depending on whether or not the photoresist is to be exposed at theparticular substrate location.

There are two methods of direct writing which can be used. The firstdirect writing method uses direct writing to essentially duplicate thelithographic mask technique. Each direct write iteration results in anaddition of a phase level to the substrate surface. A second method ofdirect writing varies the expose energy of the exposing beam as afunction of position. In other words, instead of simply turning the beamon or off, the beam energy can be modulated to the correct gray scalesin the photoresist. This process will result in a photoresist profilethat has as many levels as the desired phase plate. If done properly, asingle etching step results in the photoresist profile being transferredonto the substrate.

FIG. 12 is a flow chart illustrating a deposition method of fabricatingthe phase plates. In this fabrication process the phase levels aredeposited onto the substrate surface, rather than etched into thesubstrate. In this process, the optical substrate is coated with a layerof photoresist. A lithographic mask is then used to selectively exposethe photoresist over the desired areas of the substrate. The photoresistis then developed, where it is removed from the exposed areas. Materialis then deposited on the surface of the substrate by one of manydeposition processes (e.g., evaporation, sputtering) to the desireddepth of one of the phase levels. The residual photoresist is thenwashed off leaving a substrate surface with two phase levels. The aboveprocedure can be repeated with a second lithographic mask which leaves asubstrate with three phase levels. The process can be continued untilthe desired number of phase levels are achieved.

Any of the above processes can be used to fabricate a master element.The master element can then be used to produce a mold from whichnumerous replicas can be made. Potential replication processes includeinjection molding, compression molding, embossing, and precision glassmolding.

Although the lenslet array described above is optically efficient, theyrequire a complex fabrication process. Consequentially lenslets areexpensive to manufacture. An optical system is therefore needed thatuses less complex, and therefore inexpensive, optical elements. Otherpreferred embodiments of the invention employ a mask element in place ofa lenslet array.

FIG. 13 is a graphical representation of a constant amplitude wavefrontinterfacing with an aperture in a mask element. The mask element 50includes an aperture or opening 56. The opening 56 may be either a slitor square, as will be described in further detail below. Upon strikingthe mask element 50 only light incident on the opening 56 passes throughthe mask element 50. The opening 56 appears to be an individual lightsource with wavefronts 70 propagating outwardly. In the image plandefined by the mask element 50, the exiting wavefront 71 is in phase,but the amplitude varies in a step fashion. As the wavefront propagatesfrom the opening 56, the wavefront shifts in phase because the varyingdistance from the opening 56 to another image plane is manifested as aphase shift.

Where there are multiple openings 56 on the mask element 50, multiplewavefronts propagate from the various opening 56 and interfere with eachother. In that case, the wavefronts of light at the image plane of thephase plate 80, now have a constant amplitude, but are out-of-phase.That is because light incident on any particular point on the phaseplate 80 has arrived from multiple openings 56 of the mask 50, each ofwhich is located at a different distance from the point of interest.

The phase plate 80 disperses the incoming wavefront into wavelengthcomponents. Illustrated are red components 82, green components 84 andblue components 86 as dispersed by the phase plate 80. The dispersedcolor components 82, 84, 86 impinge on respective sub-pixels 92, 94, 96of an LCD panel 90.

FIG. 14 is a schematic diagram of a transmissive display systememploying a mask element. The display system 100 includes a light source105, a masked plate 110, a phase plate 120, an LCD panel 130. Inaddition, other optical elements can be disposed between the lightsource 105 and the phase plate 120. The light source 105 is preferably alamp with a parabolic reflector. The parabolic reflector insures thatthe light source 105 generates parallel light rays. The parallel lightrays impinge on the masked plate 110 and the wavefronts of light fromthe masked plate 110 impinge on the phase plate 120.

As described above, the phase plate 120 separates the impinging lightinto red, green and blue wavelength components and directs thosewavelength components onto respective sub-pixels of the liquid crystaldisplay panel 130. By driving the liquid crystal display panel 130 withcontrol circuitry (not shown), the sub-pixels can be valved so as togenerate a color image across the face of the display panel 130. Thedisplayed image can either be projected onto a viewing surface ordirectly viewed by a viewer.

FIG. 15 is a foreshortened cross sectional schematic diagram of themasked plate 110 of FIG. 14. The masked plate 110 includes a glasssubstrate 112 having an alternating pattern of mirrors 114 and openings116 on the distal side (relative to the light source 105). The mirrors114 can alternatively be placed on the proximal side of the glasssubstrate 112 (relative to the light source 105). The glass substrate112 can be heat absorbing, liquid cooled, or coated on either side withan infrared and ultraviolet rejection filter. The glass substrate 112can also be coated on either side with dichroic notch filters.Preferably, there is also a color rejection filter to reflect or absorbwavelength of light that are not desired for display, such as an orangeand yellow rejection filter. Although the color rejection filter can beplaced anywhere before the LCD panel 130, it is preferably integratedwith an infrared and ultraviolet filter as a single filter.

As illustrated, the linear ratio of the area of the openings 116relative to the mirrors 114 is 50%. Consequently, in the illustratedsection only one third of the light exiting the light source 105 canexit the masked plate 110 on the first attempt. The mirrors 114 reflectthe light back into the light source 105 to be reclaimed and reflectedoff from the parabolic reflector, thereby increasing the chance that anindividual light ray eventually exits an opening 116 instead of beingdissipated within the light source 105 and mask element 110.

FIGS. 16A-16B are schematic front views of the masked plate 110 of FIG.14. FIG. 16A illustrates a masked plate 110′ having slit openings 116′between mirror bars 114′. FIG. 16B illustrates a masked plate 110″having, a mirror 114″ perforated with square openings 116″. Regardlessof which embodiment is used, there is always a 2:1 ratio of mirrors 114to openings 116 in either or both directions. In the one-dimensionalgrating of FIG. 16A, one third of the light from the light source 105(plus any reclaimed light) is passed through the masked plate 110′. Forthe two-dimensional grating of FIG. 16B only one ninth of the light fromthe light source 105 (plus any reclaimed light) exits from the maskedplate 110″. The efficiency of the attempt to reclaim light is dependenton the quality of the mirrors 114 on the masked plate 110 and thereflectivity and shaping tolerances of the reflector of the light source105.

Although less light escapes from the two-dimensional grating of FIG.16B, that light can be concentrated onto individual pixel areas of theliquid crystal display panel 130. The one-dimensional grating of FIG.16A, however, wastes light by placing light energy in the black areasbetween the pixel electrodes. The choice between a one-dimensionalgrating and a two-dimensional grating is thus a system tradeoff betweenlight collection efficiency and transmission efficiency through the LCDpanel 130.

Preferably, the openings 116 in the masked plate 110 are much largerthan the wavelengths of the light. Consequently, the diffraction oflight from the openings 116 is limited. However, the 2:1 ratio of mirrorto openings efficiently handles any diffraction by assuring that thewavefronts mesh together on the phase plate 120. That is because thediffractive orders from the mask element 110 match the phase gratings onthe phase plate 120. Although a 1:2 ratio is illustrated for a threecolor display, a 1:1 ratio can be used for a monochrome display. Itshould be noted that there is a fundamental limit of a 1:3 ratio betweenopenings and mirrors, which allows the use of four different areas ofthe light spectrum such as red, green, blue and black colors in adisplay.

FIG. 17 is a perspective cross sectional schematic diagram of anotherpreferred embodiment of the invention employing a reflective box lightsource as a backlight. Fluorescent lamp elements 215 (or anothersuitable light source) are encased within a reflective box 217, which,for example, can be either a specular reflector having a mirrored insidesurface or a non-specular reflector having a highly reflective whiteinside surface. Light from the reflective box 217 passes through amasked plate 210, which is fabricated as described above. A phase plate220 is separated from the masked plate 210 and an LCD panel 230 isseparated from the phase plate 220. The reflective box 217 for a LCDpanel having a pixel pitch of 16-24 μm is less than 0.42 cm inthickness, the phase plate 220 is about 0.7 mm in thickness and the LCDpanel 230 is about 1.5 mm thick. The spacings between the elements areon the order of 1 mm with a tolerance of about 0.025 mm. The LCD panel230 can include one or two polarizers. As with the embodiment of FIG.14, the reflective box display system 200 can include a viewing lens forhead-mounted or other direct viewing of the image formed on the LCDpanel 230.

FIGS. 18A-18B are foreshortened cross sectional views of the maskedplate 210 of FIG. 17. As illustrated the masked plate 210′, 210″ iscoated with a diffusing surface to scatter incident light from thereflective box 217. The diffusing coating helps to more evenlydistribute the light from the reflective box 217. As illustrated, thediffusing coating 218 can be on the proximal side of the glass substrate212 (FIG. 18A) or on the distal side of the glass substrate 212 (FIG.18B). The defusing coating can also be on either side of the mirrors. Ineither embodiment, the mask includes alternating mirrors 214 andopenings 216 as described above; the openings 216 can either beone-dimensional slits or two-dimensional squares. In either case, thereis a 1:2 ratio of openings 216 to mirrors 214, in either or bothdirections. As above, the glass can be heat absorbing, coated on eitherside with infrared and ultraviolet rejection filters, or coated oneither side with dichroic notch filters.

A continuing problem is the need to evenly distribute light across theface of the LCD panel. One approach to solving this problem is to use alight pipe as will be described in more detail below.

FIG. 19 is a schematic diagram illustrated a preferred embodiment of adisplay system having a light pipe. The display system 300 includes aparabolic light source 305 which is a lamp with a parabolic reflector.Alternatively, the light source can include a elliptical reflector.Where the light source 305 includes a parabolic reflector a condenserlens 310 is used to focus the light from the light source 305 through anaperture 324 of a light pipe assembly 330. The aperture 324 is formed ina mask 320 having blocking elements 322. The light pipe terminates at anabutting phase plate 340 as described above. The image plane of a LCDpanel 350 is separated from the image plane of the phase plate 340 by adistance z. A projection lens 360 projects the image form on displaypanel onto a viewing surface and disposed at the center of the openingto the light pipe 330.

The light pipe assembly 330 includes four reflective surfaces 332 whichcooperate to create a light guide or pipe. The light pipe is dimensionedso as to evenly mix the light from the aperture 324 before the lightreaches the phase plate 340. The light pipe is a distance l long and hasa width w. The ratio of the width w to the length l is${{\tan (\varphi)} = \frac{w}{2l}},$

where φ is the angle from the end edge of the light pipe 330 and thecenter of the aperture 324. However,${\frac{w}{2l} = \frac{T^{\prime}}{z}},$

when T′ is the distance between pixel centers. The length of the lightpipe 330 is thus $l = \frac{wz}{2T^{\prime}}$

The aperture is preferably square with a dimension d which is defined as$d \leq \frac{w}{3}$

The light pipe 320 is preferably a rod having a rectangular crosssection and fabricated from optical glass with reflective surfaces onthe peripheral surface of the rod. The output ratio of the light pipe320 must match the square aperture 324 at the proximal end and therectangular phase plate 340 at the distal end.

FIG. 20 is a schematic diagram of a preferred embodiment of a displaysystem having multiple light sources. As described above, the displaysystem 400 includes a light pipe assembly 330 with a phase plate 340, anLCD panel 350 and a projector lens 360.

Unlike the embodiment of FIG. 19, the display system 400 of FIG. 20includes multiple light sources 405 which can be used in a linear orrectangular array. The use of multiple lamps permits the use of small,efficient, and less expensive light sources. Furthermore, the lightoutput of the system can be tuned by varying the number of lamps in thearray. Illustrated are three lamps 405 a, 405 b, 405 c. Depending on thetype of reflector used in the light sources 405, a matching lens array408 (shown in phantom) may be needed to collimate the light intoparallel light rays. As illustrated, there is one lens 408 a, 408 b, 408c for a respective light source 405 a, 40 b, 405 c. A condenser lens 310focuses the parallel light rays from the light sources 405 onto theaperture 324 and subsequently through the light pipe 330.

FIG. 21 is a schematic diagram of a preferred embodiment of a displaysystem having a changed aspect ratio. The display system 500incorporates a light pipe assembly 530 which changes the aspect ratiofrom a square aperture 524 to a rectangular face plate 340. In all otherrespects a display system 500 is identical to the display system 300 ofFIG. 19.

The above described light pipe application requires a relatively longlight pipe assembly, which restricts their application with head-mounteddisplay systems. Instead, they are primarily useful on projectiondisplay systems. The length of the light pipe, however, can be reducedby increasing the number of apertures into the light pipe.

FIG. 22 is a schematic diagram of a preferred embodiment of displaysystem having a multiple aperture light pipe in accordance with theinvention. The display system 600 includes a light source 305, acondenser lens array 610 and a light pipe assembly 630. The light pipeassembly 630 includes a mask 620 having four apertures 624 a, 624 b, 624c, 624 d. There is one condenser lens 610 a, 610 b, 610 c, 610 d for arespective aperture 624 a, 624 b, 624 c, 624 d. The light pipe assembly630 has a length l′ which is equal to l/4. This is because the fourapertures 624 a, 624 b, 624 c, 624 d of the mask 620 result in acorresponding decrease in the length of the light pipe 630.

It should be noted that both the thickness and performance of the phaseplate/LCD system are improved for displays with greater pixel densities,i.e., smaller pixels. Active matrix LCD technologies most suited to thephase plate technique are single-crystal silicon and polycrystallinesilicon, either one in transmission or reflection mode. A furtherbenefit of the phase plate technique is that the transmission of atransmissive active matrix LCD can be maintained even as the pixel sizegets smaller. As the pixel size gets smaller, the optical aperture ofthe pixel is generally reduced. The phase plate technique concentratesthe light onto the smaller optical aperture, allowing more light to betransmitted, offsetting the reduction in the optical aperture.Similarly, the phase plate technique is effective for reflective systemswhere only a fraction of the pixel area can be used to reflect theincident light.

As the pixel dimension decreases, the required distance between thephase plane and the image plane of the pixels also decreases. As such,small display panels with dense pixel arrays can provide color imageswith only a small cost in thickness to the display system. In preferredembodiments of the invention, the phase plate adds 1-2 mm to thethickness of the optics. Such systems are particularly useful wherespace is limited, such as in compact projection systems and head-mounteddisplay systems.

Although the phase element has been previously illustrated as beingseparate from the LCD panel, that is not required to practice theinvention. In another preferred embodiment the phase element can befabricated within a housing for the LCD panel on the light incidentside.

FIG. 23 is a partial cross-sectional view of an active matrix colordisplay device in accordance with the invention. Light 1005 from a lightsource is illustrated as impinging on the active matrix display device.Illustrated is one sub-pixel region of a pixel of the active matrixdevice. The display device is preferably fabricated using a lift-off andtransfer process. A matrix housing 1000 provides structural support tothe display device.

A phase plate 1010 includes a grating surface 1012 on a gratingsubstrate 1014. The phase plate 1010 is bonded to a transfer substrate1020 by a first adhesive layer 1015. A circuit substrate 1040 ofinsulating material such as SiO₂ is bonded to the transfer substrate1020 by a second adhesive layer 1025. A sub-pixel element 1030 having asub-pixel transistor 1032 and a sub-pixel electrode 1034 are formed inor on the circuit substrate 1040. The transistor is fabricated from alayer of single crystal silicon 1033 and the electrode 1034 isfabricated from a light transmissive material, such as single crystalsilicon, polcrystalline or ITO. A passivation layer 1035 of SiN, forexample, is formed over the sub-pixel element 1030. A detaileddescription of fabricating such sub-pixel elements 1030 is provided inU.S. Pat. No. 5,377,031, issued on Dec. 27, 1994 to Vu et al. and U.S.application Ser. No. 08/215,555, entitled “Method of Fabricating ActiveMatrix Pixel Electrodes,” and filed on Mar. 21, 1994 by Zavracky et al.,the teachings of which are incorporated herein by reference. Alsoillustrated is a contact metalization 1038 and a light shield (e.g.,black matrix) element 1045.

A layer of liquid crystal material 1050 is disposed between thetransistor 1030 and a counterelectrode 1062 on a glass substrate 1064. Apolarizer 1070 is bonded to the counterelectrode substrate 1064 by anadhesive layer 1065. An anti-reflection coating 1080 is formed on thepolarizer.

In preferred embodiments of the invention, the phase changes imparted bythe phase plate do not necessarily have to be produced by an air-surfaceinterface. The phase changes can occur in a boundary layer, where theindex of refraction of the boundary layer is a function of position.Various techniques can be used to fabricate such a structure. Forexample, an optically sensitive layer such as dichromated gelatin can bedeposited on a substrate and selectively exposed as a function ofposition. The expose step can be done using the masking technique, thedirect writing technique or a holographic technique. After exposure, theindex refraction of a particular location on the optically sensitivelayer becomes a function of the optical exposure at that location.

Preferably, the display systems employing light pipes are integratedinto a projector housing to create a projection display device. However,the length of the light pipe can be quite long where there is a singleaperture. To make the projection display device more manageable, thelight pipe must be either shortened as illustrated in FIG. 22 or folded,as described below.

FIG. 24 is a schematic diagram of a preferred embodiment of theinvention embodied in a display system having a folded light pipeillumination system. The projection display system 700 includes aprojector housing 702 in which the optical elements are enclosed. Lightis provided by a light source 305, which is preferably a lamp with aparabolic reflector. The parallel light rays from the light source 305are focused by a condenser lens 310 onto an aperture 324 of a mask 320.The light from the aperture 324 enters a folded light pipe 730, whichmixes the light presented to a phase plate 340. The phase plate 340disperses the incoming wavefront into red, green and blue wavelengthcomponents and directs those wavelength components onto sub-pixels of anLCD panel 350. A control circuit 770 drives the LCD panel 350 to createa display image which is projected by a projection lens 360 onto aviewing surface. A fan 780 is also disposed within the projector housing702 to dissipate thermal energy from within the housing 702. As colorfilters are not being used, the thermal management demand on the coolingsystem can be reduced to provide for a more compact housing enclosure.

As illustrated, the light pipe 730 is folded to create a right-anglebend. The light pipe 730 comprises three optical subcomponents: a firstlight pipe subcomponent 731, a second light pipe subcomponent 733, and aprism subcomponent 735. The light pipe subcomponents 731, 733 includemirrored surfaces 732, 734. Likewise, the prism subcomponent 735includes mirrored surfaces 736 on all sides. Although only a singleprism 735 is illustrated to create one fold in the light pipe assembly730, additional prisms can be employed to create multiple folds in thelight pipe.

In the above embodiments, a light pipe has been described as mixing thelight from the light source and matching the aspect ratios between themask aperture and the phase plate. The light pipe, however, need not beused if the light source itself generates a well mixed light beam. Onesuch light source has spherical reflector.

FIG. 25 is a schematic block diagram of a preferred embodiment of theinvention embodied in a projection display system having a sphericalreflector light source. The projection display system 800 includes aprojector housing 802 in which the optical components are enclosed. Aspherical light source 805 having a lamp and a spherical reflector ischosen to produce a well-mixed light beam.

The light beam from the spherical reflector is collected by a field lens810 and directed toward a mirror 820. A mirror 820 is preferablyoriented at 45° angle relative to the incoming light beam so as tocreate a 90° bend in the light path. Preferably, the mirror 820 is acold mirror. The light beam then passes through a lenslet array 830,where the light is focused onto a phase plate 840 as described above.The light from the phase plate 840 is valved by an LCD panel 850 underthe control of control circuitry 870 to create a color image. The colorimage is projected onto a viewing surface by a projection lens 860. Afan 880 is also disposed within the projector housing 802 to dissipatethermal energy from the projection display system 800.

FIG. 26 is a schematic diagram of a preferred embodiment of theinvention embodied in a projection monitor. The projection monitor 910includes an illuminated LCD panel 918 and an optical arrangement 920 fordirecting the light beam from the LCD panel 918 to a screen 930. Theprojection system can include any of the aforementioned projectiondisplay system; particularly those using lamps as a light source.Further embodiments of projection monitors which can incorporate theinvention are described in U.S. patent application Ser. No. 08/015,813,entitled “Projection Monitor” and filed on Feb. 10, 1993, the teachingsof which are incorporated herein by reference.

As illustrated, a spherical light source 912 directs light to a lens 914which converges the light onto a lenslet array 916. Light from thelenslet array 916 is focused onto a phase plate 917. A dispersed lightfrom the phase plate is focused onto the display region of an LCDdisplay panel 918. As described above, the phase plate 917 can dispersethe light into monochrome light or color light.

The image light from the LCD panel 918 is directed through a collectinglens subsystem 922 onto a first mirror 924. The image light is thenreflected off of first mirror 924 through a projection lens subsystem926. Light is then re-reflected off of a second mirror 928, which formsthe back of a monitor housing. The light from the second mirror 928 isthen presented to a viewer on a viewing screen 929, which forms thefront of the monitor housing.

FIG. 27 is a schematic diagram of another preferred embodiment of theinvention embodied in a projection monitor. The projection monitor 930includes a monitor housing 932 securing a back mirror 942 and a viewingscreen 944. Light from a spherical light source 934 is collected by acollecting lens 935 for delivery to a lenslet array 936. The lenslets936 focus the light onto pixel regions of a phase plate 937. Thedispersed light from the phase plate 937 impinges on individualsub-pixels of a display panel 938, which valves the light to create animage. The images are focused by a projection lens 939 onto a frontprojection mirror 940. The light is reflected off from the frontprojection mirror 940 onto the back mirror 942, which in turns reflectsthe light onto the viewing screen 944.

The above projection display devices can include a user interface. Forexample, the display devices can include either housing-mounted orremotely-mounted control buttons, such as for adjusting brightness,contrast, color, and focus. The display device can also include a motoractuated zoom lens, which can be remotely controlled by the user. Thedisplay device can also include speakers with user-controllable balance,tone and volume. A storage device such as a disk drive or CD-ROM playerand a computer interface can be provided to complete a multimediasystem.

FIG. 28 is a schematic diagram, shown partially in cross-section, of apreferred embodiment of the invention embodied in a direct-viewhead-mounted display device 950. Shown is a display housing 952 in closeproximity to an eye of a user. The display housing 952 is preferablyfabricated from plastic, but other lightweight materials can also beused. As illustrated, the display housing 952 is coupled to a mountingarm 953 by a swivel joint 959. The mounting arm connects to head-mountedframe to form a monocular display device.

A backlight 955, such as the lamp elements 215 of FIG. 17, is mounted ina lamp housing 954 which includes reflective inner surfaces. Light fromthe backlight 955 is projected through a lenslet array 956, a phaseplate 958 and a display panel 960 to form an image.

The display system also includes a polarizing filter 962, asemi-reflective concave mirror 964, and a cholesteric liquid crystal(CLC) element 966. The image that is generated by the display panel 960is transmitted through the filter 962, the semi-reflective concavemirror 964 to the CLC element 966. The CLC element 966 reflects theimage back onto the mirror 964 which rotates the light so that, uponreflection back to the CLC element 966, the light is transmitted throughthe CLC element 966 to the viewer's eye. A lens can be used with thissystem depending upon the size, resolution, and distance to the viewer'seye of the optical system components and the particular application. Afocus adjust mechanism can also be provided for use by the user.

FIG. 29 is a schematic diagram, shown partially in cross-section, of apreferred embodiment of the invention embodied in a projectionhead-mounted display device 970. Shown is a plastic display housing 972in close proximity to the eye of the user. As illustrated, the displayhousing 972 is coupled to a head-mounted frame 973 to form one-half of abinocular display device.

A backlight 975, such as the lamp 215 of FIG. 17 is mounted in a lamphousing 974 which includes reflective inner surfaces. Light from thebacklight 975 is projected through a lenslet array 976, a phase plate978 and display panel 980 to form an image. The image is reflected offfrom a mirror 982, through a viewing lens 984 and a cover glass 986 tobe viewed by the user.

Further embodiments of head-mounted display devices which canincorporate the invention are described in U.S. patent application Ser.No. 08/327,113, entitled “Head-Mounted Display System” and filed on Oct.21, 1994, the teachings of which are incorporated herein by reference.

FIG. 30 is a schematic diagram, shown partially in cross-section, of apreferred embodiment of the invention embodied in a projection displaysystem having a virtual light source. Like previous embodiments of theinvention, a light pipe 330 is used to mix light prior to the lightpassing through a phase plate 340. The phase plate is registered to alight valve display panel 1150 which is electronically controlled toform an image for projection through a projection lens 360. Unlikepreviously described embodiments of the invention, the light valvedisplay panel 1150 does not include a front polarizer, only a backpolarizer. The incident light is polarized, instead, at the lightsource.

Light from a lamp 1102 is split by a broadband polarizing beamsplitter1104 into polarized light, which passes through the beamsplitter 1104and unpolarized light which is reflected at the interface 1105. Thepolarized light is focused by a main focusing lens 1110 and an optionalsecondary focusing lens 1108 a onto the aperture 324 of the light pipe330.

The unpolarized light is recovered by a recovery optic to create avirtual light source. The unpolarized light reflected from the interface1105 is reflected by a recovery mirror 1106 so that the light passesthrough a one-quarter wave plate 1107. The wave plate 1107 polarizes thewaste light from the beamsplitter 1104, which is in turn directed to theaperture 324 of the light pipe 330 by the main focusing lens 1110 and anoptional secondary focusing lens 1108 b.

An unpolarized beam of light from the lamp 1102 is thus split into twobeams of different and orthogonal polarizations. The reflected wastebeam is redirected with the recovery mirror 1106 and converted via thequarter-wave plate 1107 to the polarization of the transmitted beam. Thespecial separation or angular separation between the two beams is chosenso the two beams are collected and transmitted through the phase plate340, where they add to one another constructively. Although only onelamp 1102 is illustrated, multiple lamps, each with a polarizingbeamsplitter, can be used to form a two-dimensional array of lightsources.

Although the above preferred embodiments of the invention have beendescribed as using transmissive liquid crystal display panels, otherdisplay panels can be used. For example, the invention can be used witha digital micromirror device (DMD), which have an array of reflectiveelectromechanical pixels (i.e., micromirrors). Light incident on eachpixel is reflected in either of two directions based on whether thepixel is on or off. Such systems are described in further detail in U.S.Pat. No. 5,382,961 and U.S. Pat. No. 5,457,493, the teachings of whichare incorporated herein by reference.

FIG. 31 is a schematic diagram of a preferred embodiment of theinvention embodied in a display system having a digital micromirrordevice (DMD). Light is created by a spherical reflector lamp 1202. Acollimating lens 1204 collects the mixed light from the sphericalreflector lamp 1202 and forms parallel rays of white light. A lensletarray 1210 is attached to a face of a total internal reflection (TIR)prism 1250 by a mounting member 1212. A phase plate 1260 and a DMD 1270are attached to another face of the TIR prism 1250 by a mounting member1265. The microlens array 1210 and the phase plate 1260 are registeredbefore being mounted to the TIR prism 1250. The registration isaccomplished by optically aligning the elements through the TIR prism1250. A preferred apparatus and method for mounting the microlens array1210 and phase plate 1260 is described in U.S. patent application Ser.No. 08/285,955, filed Aug. 4, 1994 by Bryan E. Loucks, the teachings ofwhich are incorporated herein by reference. Other suitable mountingmembers can also be used. In turn, the sub-pixels on the DMD 1270 areregistered to the phase plate 1260. The phase plate 1260 and the DMD1270 can also be fabricated as a single element where the registrationis performed during the fabrication process.

Briefly, the TIR prism 1250 is configured to totally reflect anillumination beam from the light source at the air gap 1255. Thereflected beams from the DMD 1270 are transmitted through the air gap1255. The thickness of the air gap 1255 is minimized to reduceaberrations in the transmitted beam. Such TIR prisms are discussed indetail in U.S. Pat. No. 4,983,032, the teachings of which areincorporated herein by reference.

Shown are the optical paths for a single light bundle representing asub-pixel of the display. An incoming light bundle of white light 1215from a lenslet of the lenslet array 1210 is reflected off of theinterface 1255 between the prism elements 1252, 1254. The light bundleof white light 1215 is dispersed by the phase plate 1260 onto red, greenand blue sub-pixels of the DMD 1270. If a particular sub-pixel is on, atransmitted color light bundle 1272 reflected from the sub-pixelmicromirror passes through the TIR prism 1250 to a projection lens 1280.If the sub-pixel is off, the resulting color light bundle 1278 reflectedfrom the sub-pixel micromirror is directed into the TIR prism 1250 anddoes not exit through the projection lens 1280.

It should be noted that light passes through the phase plate 1260 twotimes. The spacing between the phase plate 1260 and the DMD 1270 must besmall enough so the phase plate 1260 does not destroy the pixelresolution of image formed by the DMD 1270. In preferred embodiments ofthe invention, the pixel pitch is between 30-60 μm with a sub-pixelpitch of between 10-20 μm. As such, the spacing between the phase plate1260 and the DMD 1270 is between about 600 μm-2 μm. For the case of thetransmitted light bundle 1272, the second pass of the light through thephase plate 1260 thus depixelizes the resulting image by spreading thereflected light from the pixel mirror over the area of the sub-pixel.

Although a spherical reflector is illustrated in FIG. 30, othertechniques to provide light to the TIR prism 1250 can be employed bydevices embodying the invention. In particular, a light pipe such asdescribed above can be used.

FIG. 32 is a schematic diagram, partially in cross-section, of apreferred embodiment of the invention embodied in a DMD projectionsystem having a light pipe. Light is provided to the TIR prism 1250through a light pipe 330 as described above. A lamp 1206 generates abeam of light which is focused by a focusing lens 1208 through theaperture 324 of the light pipe 330. After entering the TIR prism, thelight behaves as discussed above with reference to FIG. 30.

FIG. 33 is a schematic diagram, partially in cross section, of apreferred embodiment of the invention embodied in a reflective liquidcrystal display system. Illustrated is a lamp 1302 having a sphericalreflector to generate well-mixed white light. The light from the lamp1302 is collimated by a lens 1304. The parallel rays of light arewavelength dispersed by a diffractive optic element 1360, fabricated asdescribed above. Each of the red, green and blue wavelength regions aretargeted onto specific sub-pixel regions of a reflective liquid crystaldisplay panel 1370.

If a sub-pixel is activated, the respective wavelength region passesthrough a respective volume liquid crystal material and impinges on areflective electrode. The wavelength region of light is then reflectedby the electrode back through the same volume of liquid crystal materialtoward a projection or viewing lens 1380. As illustrated, the light isincident on the diffractive optic element 1360 and the display panel1330 at an angle α relative to the normal axis.

Because reflective electrodes are used, the controlling transistors foreach sub-pixel electrode can be fabricated beneath the electrode. Suchan arrangement allows for a smaller pixel pitch and full usage of thesurface of the display panel 1370 without requiring mechanical elements.The display device can therefore be more compact than transmissionliquid crystal displays.

Although certain combinations of housings and optical configurationshave been illustrated and described, the scope of the invention is notlimited to those particular combinations. The housings and opticalconfigurations can be interchanged and different housings can beemployed with any optical configuration, including those opticalconfigurations which are shown without a housing.

Equivalents

Those skilled in the art will know, or be able to ascertain using nomore than routine experimentation, many equivalents to the specificembodiments of the invention described herein. These and all otherequivalents are intended to be encompassed by the following claims.

This invention claimed is:
 1. A method of imaging using an active matrixliquid crystal display device comprising: providing an active matrixarray of transistor circuits and pixel electrodes, each pixel electrodebeing electrically connected to a transistor circuit that actuates apixel electrode; providing a layer of liquid crystal material positionedbetween the pixel electrodes and a counter electrode to form a displaystructure such that actuation of the pixel electrodes controlstransmission of light through a volume of the liquid crystal material;providing a diffractive optical element optically coupled with theactive matrix array to disperse light of different colors throughdifferent volumes of the liquid crystal material; directing light from alight source through the display device while selectively actuating thepixel electrodes to form an image with the display device; andmagnifying the image with a lens.
 2. The method of claim 1 furthercomprising providing a device wherein the diffractive optical element isbonded to the active matrix array.
 3. The method of claim 1 furthercomprising providing a diffractive optical element that includes a lightpolarizer.
 4. The method of claim 1 further comprising providing ahousing that forms a direct-view display device.
 5. The method of claim1 further comprising providing a housing that forms a projected-viewdisplay device.
 6. The method of claim 1 further comprising providing anillumination system that includes a light pipe.
 7. The method of claim 6wherein the step of providing a light pipe further comprises providing afolded light pipe.
 8. The method of claim 6 further comprising providingan illumination system that includes a lenslet array, the lenslet arraybeing optically aligned with the diffractive optical element.
 9. Amethod of imaging using an active matrix liquid crystal display devicecomprising: providing a light source that emits red, blue and greencolors and that is optically coupled to an active matrix array oftransistor circuits and pixel electrodes, each pixel electrode beingelectrically connected to a transistor circuit that actuates a pixelelectrode; providing a layer of liquid crystal material positionedbetween the pixel electrodes and a counter electrode to form a displaystructure such that actuation of the pixel electrodes controlstransmission of light through a volume of the liquid crystal material;providing a diffractive optical element optically coupled with theactive matrix array to disperse light of different colors throughdifferent volumes of the liquid crystal material; directing light fromthe light source through the diffractive optical element and the displaydevice while selectively actuating the pixel electrodes to form an imagewith the display device; and magnifying the image with a lens.
 10. Themethod of claim 9 further comprising providing a device wherein thediffractive optical element is bonded to the active matrix array. 11.The method of claim 9 further comprising providing a diffractive opticalelement that includes a light polarizer.
 12. The method of claim 9further comprising providing a housing that forms a direct-view displaydevice.
 13. The method of claim 9 further comprising providing a housingthat forms a projected-view display device.
 14. The method of claim 9further comprising providing an illumination system that includes alight pipe.
 15. The method of claim 14 further comprising providing alight pipe further comprises providing a folded light pipe.
 16. Themethod of claim 14 further comprising providing an illumination systemthat includes a lenslet array, the lenslet array being optically alignedwith the diffractive optical element.
 17. A method of imaging using anactive matrix liquid crystal display device comprising: providing alight source that emits red, blue and green colors and that is opticallycoupled to an active matrix array of transistor circuits and pixelelectrodes, each pixel electrode being electrically connected to atransistor circuit that actuates a pixel electrode; providing a layer ofliquid crystal material positioned between the pixel electrodes and acounter electrode to form a display structure such that actuation of thepixel electrodes controls transmission of light through a volume of theliquid crystal material; providing a diffractive optical element alignedwith the active matrix array to disperse light of different colorsthrough different volumes of the liquid crystal material; and directinglight from the light source through the diffractive optical element andthe display device while selectively actuating the pixel electrodes toform an image with the display device.
 18. The method of claim 17further comprising providing a device wherein the diffractive opticalelement is bonded to the active matrix array.
 19. The method of claim 17further comprising providing an illumination system that includes alenslet array, the lenslet array being optically aligned with thediffractive optical element.